To perform RF filtering, SAW (surface acoustic wave) filters may be used, which are produced by coupled SAW resonators. Typically, for such a filter with dimensions equal to about 3 mm×3 mm×1 mm, the insertion losses can be between 2.5 dB and 3 dB with a rejection equal to about 30 dB. However, these filters have limitations. The maximum resonance frequencies are generally equal to about 3 GHz, and the maximum power handling is equal to about 1 W. Outside of this range of operation, SAW devices have significant propagation losses.
BAW (bulk acoustic wave) filters may be produced from piezoelectric BAW resonators coupled electrically (with a ladder or a lattice structure, for example) or acoustically (of the (SCF) Stacked Crystal Filter type or the (CRF) Coupled Resonator Filter type). In such a filter, the signal to be filtered is propagated vertically in stacked resonant layers, directly or by an acoustic propagation medium, one on top of another. The dimensions and insertion losses capable of being obtained with these BAW filters are comparable to those of the SAW filters. However, the power handling of these BAW filters can reach about 3 W and the maximum resonance frequencies can be greater than about 16 GHz. Finally, the production of these filters is compatible with CMOS and BiCMOS technologies.
FIG. 1 shows an example of a BAW resonator 1 comprising a layer 2 based on a piezoelectric material, a lower electrode 4 and an upper electrode 6. The dimensions of the lower electrode 4 and the piezoelectric layer 2 are substantially similar to one another in a plane parallel to plane (x, z) based axes x and z shown in FIG. 1. The lower electrode 4 is then completely covered by the piezoelectric layer 2. In contrast, the upper electrode 6 has a shape and dimensions different from those of the lower electrode 4.
In the example of FIG. 1, the upper electrode 6 has a dimension according to axis x that is smaller than that of the lower electrode 4. This difference in dimensions between the lower electrode 4 and the upper electrode 6 forms two zones in the piezoelectric layer 2. A first active zone 8 is the piezoelectric layer 2 contained between the two electrodes 4 and 6. A second inactive zone 10 is the piezoelectric layer 2 arranged on the lower electrode 4 without being covered by the upper electrode 6. The speed of propagation of the waves in the active zone 8 is different from that in the inactive zone 10. This difference in propagation speed results in parasitic resonances due to the propagation of lateral waves perpendicular to the vertical waves, called Lamb waves. The energy of the Lamb waves is proportional to the value of the difference between the bulk acoustic propagation coefficients (propagation according to axis y) of the active zone 8 and the inactive zone 10.
U.S. published patent application no. 2006/0076852 describes electroacoustic components using bulk acoustic waves. Electrodes are positioned periodically on a piezoelectric layer so as to guide the bulk acoustic waves into the component. The value of the piezoelectric coefficient of this layer at the level of the electrodes is different from that at the level of the portions of the layer not covered by the electrodes. This difference with regard to the propagation coefficient in the piezoelectric layer is difficult to obtain and requires specific steps of treating the piezoelectric layer.
The article “UHF/VHF resonators using Lamb waves co-integrated with Bulk Acoustic Wave resonators” by A. Volatier et al., IEEE Ultrasonics Symposium, 2005, pages 902 to 905, describes Lamb wave resonators comprising a square or rectangular electrode. The order of the resonance mode is chosen according to the resonance frequency desired. These resonators have, in particular, the disadvantages of having a relatively low quality factor and a high series resistance.